The for Loop • Another loop statement, for, is best for when you can determine in advance how many times you need to execute the loop (counting loop). For Loops • The for statement includes the three parts needed for loops: initialize, test, and update. 15-110 Summer 2010 • All this information is conveniently placed at the beginning of the loop. Margaret Reid-Miller • All three loop statements (while, do, and for) are functionally equivalent. Summer 2010 15-110 (Reid-Miller) 2 The for statement The for statement • The form of the for statement is • The form of the for statement is for (<initialize>; <boolean_expression>; <update>) for (<initialize>; <boolean_expression>; <update>) <statement> <statement> • First, the initialize statement is executed. • It is equivalent to • If boolean_expression evaluates to true, then statement (body of loop) is executed, followed by the <initialize>; update statement. while (<boolean_expression>) { <statement> • The loop repeats until the boolean_expression <update>; executed after evaluates to false. } statement (body of loop) Summer 2010 15-110 (Reid-Miller) 3 Summer 2010 15-110 (Reid-Miller) 4 The for Flowchart A for Loop Example initialize n = 4 !int sum = 0;! sum i !for (int i = 1; i <= n; i++) {! 0 false ! !sum += i*i;! 1 boolean_expression i <= n ? !}! 1 !System.out.println(sum);! 2 update true i <= n ? Which variable is the loop 5 3 statement control variable? i <= n ? 14 (body of loop) 4 i <= n ? 30 5 i <= n ? Summer 2010 15-110 (Reid-Miller) 5 Summer 2010 15-110 (Reid-Miller) 6 Another for Loop Example Scope n = 11 sum i • The scope of a variable is the area within a program that !int sum = 0;! 0 can reference the variable. !for (int i = 1; i <= n; i+=3) {! 1 • The scope depends on where the variable is declared.! ! !sum += i;! i <= n ? 1 !}! !int sum = 0;! 4 !System.out.println(sum);! i <= n ? !for (int i = 1; i <= n; i++) {! Scope of 5 ! !sum += i*i;! 7 variable i i <= n ? !}! 12 !System.out.println(sum);! 10 i <= n ? 22 13 i <= n ? Summer 2010 15-110 (Reid-Miller) 7 Summer 2010 15-110 (Reid-Miller) 8 Scope Nested Loops !int sum = 0;! • A loop can have another loop inside of it. !int i;! • For each iteration of the outside loop, the inside loop !for (i = 1; i <= n; i++) {! runs completely. ! !sum += i*i;! Scope of • Often it is easiest to read from the inside out. !}! variable i • Example: !System.out.println(“Sum of first “ + (i-1)! ! + “ integers squared is “ + sum);! How many lines are printed? for (int i = 1; i <= 5; i++) {! for (int j = 1; j <= 3; j++) {! System.out.println(i + " " + j);! }! What happens if we } write println(i + j)? Summer 2010 15-110 (Reid-Miller) 9 Summer 2010 15-110 (Reid-Miller) 10 Palindromes Which Loops? • A palindrome is word, phrase, or sequence that reads • for loops are more natural when we know how many the same backwards as forwards. iterations we need (definite or counting loops). Examples: • Example: Bob by Weird Al Yankovic • Print "*" 10 times • Print the even numbers between 10 and the value of n (A parody of Bob Dylan's Subterranean Homesick Blues) • while and do loops are more natural when we want to keep looping until some outcome (indefinite or result controlled loops). http://www.youtube.com/watch?v=Nej4xJe4Tdg Examples: • Prompt the user until the user inputs the data in the correct How would you test whether a string is a palindrome? form. • Continue looping until we reached a million dollars. Summer 2010 15-110 (Reid-Miller) 11 Summer 2010 15-110 (Reid-Miller) 12 .